This invention relates to apparatus for melting and dispensing thermoplastic materials such as hot melt adhesives. This type of thermoplastic material is received in the form of an elongated, flexible rod, as disclosed, for example, in U.S. Pat. No. 2,874,084, issued Feb. 17, 1959, in the name of Hans C. Paulsen. The invention disclosed may be utilized in portable hand-operated cement extruding guns such as used for home shop use or light industrial use and disclosed, for instance, in U.S. Pat. No. 3,743,142, issued July 3, 1973, in the names of Richard M. Elliott and Albert E. Newton. The invention may also be adapted to heavier industrial usage, for example, in applying adhesives in the manufacture of shoes or in the production of package containers.
In general, the glue dispensing appliance in which the subject system is used consists of a gun shaped device having a handle and a barrel. A heat radiating body (melt body) is constructed within the barrel to transfer heat from electric heaters to a melt chamber. Means are provided in the handle to feed a flexible rod of heat activatable glue to the melt chamber. As the glue is melted it is forced out of a nozzle at the exit of the melt chamber. A thermostat or other heat sensing device is used to prevent overheating and maintain the desired temperature of the melt chamber.
In the normal operation of this type of appliance, the heaters are energized and allowed to reach operating temperature. The flow of glue is generally initiated intermittently as needed without shutting the heat off between uses. The glue gun may, therefore, sit idle for extended periods while the desired temperature is maintained by a suitable sensor-control.
When the glue rod is being fed into the melt body, it maintains a gradually diminishing solid core about which melted glue flows. The melt chamber and entrance are filled with material. When the glue gun is placed in idle condition with the heat on, but with no glue entering, the glue within the melt body melts and expands. Since the space is limited and filled, an increase in pressure results. This pressure could cause leakage through the ball valve in the nozzle or back through the control housing neither of which is desirable. In order to avoid this problem, means are provided to retract the solid core of the glue rod when the glue feed is stopped, thereby creating space in the melt body to allow for the expansion of the glue as it increases in temperature under idling condition without an increase in pressure.